The subject invention concerns a method of manufacturing compacted graphite cast iron/vermicular cast iron by adding graphite-modifying alloying agents, a so-called treatment alloy, to low-sulphurous molten cast iron.
Several methods are used today to manufacture compacted graphite cast iron. The most well-known method is the ladle treatment, according to which treatment an alloy, usually consisting of FeSiMgRECa, wherein RE refers to rare earth metals, is reacted with the iron. Several varieties of ladle treatments exist, such as the sandwich, the tundish cover, and other varieties. However, all varieties suffer from a number of deficiencies, common to them all. One is the extensive manual handling that often is required, such as slagging and tapping operations. Another drawback is the considerable length of time between the treatment and the casting operations that the ladle method in its different varieties requires, in addition to which the length of this time as also that of the casting often vary. This is a serious drawback since, preferably, compacted graphite cast iron should be cast after a holding time that should be constant and not exceed 5 minutes following the treatment. The reason therefor is the unstable condition of the molten iron during the treatment. Immediately following the treatment, the Mg and RE elements therein start to fade out, i.e. to evaporate and to form sulphide and oxide at different paces. Among other things, this makes it considerably more difficult to maintain the appropriate temperature in the casting oven, and as a results thereof, handle the iron rationally in a continuous-casting foundry. The contents of Mg and RE must be maintained within very narrow intervals, since the structural variations allowed in the castings are very limited.
In order to achieve correct structure and quality in the castings the fade-out during the temperature maintenance period, i.e. the Mg and RE contents and the nucleating condition, must be continuously measured and the conditions of the treatment be corrected. No reliable and productional technology known to the present inventors exists, either for measuring or correcting these parameters. This obviously very difficult problem of process control is one of the main reasons for the lack of success of compacted graphite iron as a constructional material, despite the excellent properties that this material possesses as such.
Another method which is based on a well-known principle is the in-mold treatment. In accordance with this method, MgRE is added to the iron by placing a treatment alloy in a reaction chamber incorporated in the casting system of the mold. When the molten iron flows over the treatment alloy in contact therewith, it picks up MgRE in amounts that are dependent on several factors, among them the flow of the iron, the area of the reaction chamber, the temperature of the iron, and the size of the granules of the alloy.
This method is advantageous, inasmuch as only the temperature of the untreated east iron need to be maintained at the correct level. Consequently, the problem of fade-out is eliminated, since the treatment lasts for a few seconds only, prior to the molten cast iron reaching the mold cavity.
However, this method requires that the casting system, the area of the reaction chamber, and other parameters be carefully designed and calculated in all details in order to ensure that the castings be entirely free of inclusions, reaction agents and undecomposed treatment alloy material, and thus that a perfect treatment result be obtained. Also, the reaction chamber encroaches on the space inside the mold and diminishes the casting yield, in that some molten iron remains in the chamber.